A number of metrology tools are now available for performing optical measurements on semiconductors. Such tools can include spectroscopic reflectometers and spectroscopic ellipsometers. Examples of such tools can be found in U.S. Pat. Nos. 5,608,526 and 6,278,519, both incorporated by reference. Such tools typically include a broadband light source for generating a probe beam that is directed to reflect off the sample. Changes in intensity or polarization state of the beam as a function wavelength are monitored to yield information about the sample.
Given the continuing shrinking feature size of semiconductor circuits, it is desirable to design the illumination system to provide a tightly focused probe beam to form a small spot on the sample surface. Several focusing assemblies have been developed for this purpose. These focusing assemblies can be formed from refractive or reflective elements or a combination of each (cadiatropic assemblies).
For patterned samples, such as integrated circuits, the metrology instrument must measure within small test pads [i.e. often less than 50 microns wide] surrounded by a completely different material or film stack. The generation of adequate and meaningful contrast can be as important as resolution. Some of the difficulties in practice are:                (a) The optics must accommodate ever-smaller feature size [critical dimension] and tighter position [overlay] requirements, for specific materials.        (b) The optics must precisely locate the test feature boundaries, since any light falling on the surrounding features and collected by the detector can cause measurement error.        (c) The optics between the source and detector must minimize the radiation falling on or detected from areas outside the smallest measurable test feature. This condition must be achieved over the instrument's entire wavelength range.        
In such systems, it is important to understand and control the range of light illumination and collection angles. The sine of the maximum illumination half angle is commonly referred to as the illuminator numerical aperture, or NAI. The sine of the maximum collection half angle is commonly referred to as the collector numerical aperture, or NAC. The numerical aperture of each branch of the illumination system is set by the smallest or limiting aperture in that branch. The limiting aperture can be a separate physical element or be part of the focusing system (lens assembly) or other optical element.
The most common practice in the art is to select NAI=NAC (typical microscope systems are set up this way). With this selection (equal angular ranges for illumination and collection systems) the collection optics only lightly apodizes the reflected probe beam and the system has high throughput. A significant shortcoming of this arrangement is reduced image contrast for small features; e.g. image contrast is inversely proportional to object spatial frequency and, although the imaging system can provide high contrast images of large features, the image contrast of small features is degraded.
It is another common practice to set NAI>NAC and even NAI>>NAC. Note that the spherical aberration and the resolution of the collection optics are proportional to NAC while the system throughput is proportional to NAC2. Decreasing NAC is advantageous since it reduces aperture dependent aberrations, e.g. spherical aberration. However, this reduction in aberrations comes at the cost of reduced throughput and degraded resolution. Reducing NAC has the further disadvantage that the collection aperture now heavily clips the reflected probe beam; this produces additional measurement error due to aperture dependent scatter and diffractive effects.
Neither of the conventional practices is completely satisfactory for small-spot optical spectroscopic measurement systems.